Calculate an estimated Synthroid dosage based on patient weight and desired TSH target.
mcg/kg
mcg/m² (Body Surface Area – requires height and weight)
Understanding Synthroid Dosage Calculation
Synthroid (levothyroxine sodium) is a synthetic thyroid hormone replacement medication used to treat hypothyroidism, a condition where the thyroid gland does not produce enough thyroid hormones. The dosage of Synthroid is highly individualized and depends on several factors, including the patient's weight, age, overall health, the specific reason for treatment (e.g., primary hypothyroidism, TSH suppression), and the target Thyroid Stimulating Hormone (TSH) level.
This calculator provides an estimated starting dose. It is crucial to understand that this is a simplified model and should NEVER replace professional medical advice. Always consult with your healthcare provider for accurate diagnosis and personalized treatment plans.
Calculation Logic
The most common method for initial Synthroid dosing is based on the patient's body weight. The general guideline for treating primary hypothyroidism is typically between 1.6 to 1.8 mcg per kilogram (mcg/kg) of body weight per day.
For specific conditions, such as TSH suppression for certain types of thyroid cancer, higher doses might be required. Conversely, elderly patients or those with cardiac conditions might start on a lower dose.
This calculator uses a common weight-based formula. It also includes an option to calculate based on Body Surface Area (BSA), which is sometimes used, especially in pediatric or specific clinical scenarios. The BSA formula used here is the Mosteller formula:
A typical starting dose based on BSA is approximately 100-150 mcg/m² per day. The TSH target influences the final dosage adjustment, but this calculator focuses on the initial estimated dose calculation.
Factors Influencing Dosage
Weight: Higher weight generally requires a higher dose.
Age: Elderly patients may require lower doses.
Cardiac Conditions: Pre-existing heart issues may necessitate a slower dose titration.
Pregnancy: Thyroid hormone needs increase during pregnancy.
Other Medications: Certain drugs can affect Synthroid absorption or metabolism.
Severity and Cause of Hypothyroidism: Primary vs. secondary hypothyroidism, and the degree of thyroid hormone deficiency.
TSH Target: The desired level of TSH suppression or normalization.
Disclaimer
This calculator is intended for informational purposes only and does not constitute medical advice. The calculated dosage is an estimate and may not be appropriate for every individual. Medication dosages must be determined by a qualified healthcare professional based on a thorough evaluation of the patient's condition, laboratory results, and clinical presentation. Do not adjust your Synthroid dosage without consulting your doctor.
document.getElementById('weightUnit').addEventListener('change', function() {
var selectedValue = this.value;
var bsaInputDiv = document.getElementById('bsaInput');
if (selectedValue === 'mcgPerM2') {
bsaInputDiv.style.display = 'block';
} else {
bsaInputDiv.style.display = 'none';
}
});
function calculateSynthroidDose() {
var weightKg = parseFloat(document.getElementById("patientWeightKg").value);
var tshTarget = parseFloat(document.getElementById("tshTarget").value);
var weightUnit = document.getElementById("weightUnit").value;
var heightCm = parseFloat(document.getElementById("patientHeightCm").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(weightKg) || weightKg <= 0) {
resultDiv.innerHTML = "Please enter a valid patient weight in kg.";
return;
}
if (isNaN(tshTarget) || tshTarget <= 0) {
resultDiv.innerHTML = "Please enter a valid target TSH level.";
return;
}
if (weightUnit === 'mcgPerM2') {
if (isNaN(heightCm) || heightCm <= 0) {
resultDiv.innerHTML = "Please enter a valid patient height in cm for BSA calculation.";
return;
}
}
var estimatedDoseMcg = 0;
if (weightUnit === 'mcgPerKg') {
// General starting dose range: 1.6 to 1.8 mcg/kg/day
var lowerBound = weightKg * 1.6;
var upperBound = weightKg * 1.8;
estimatedDoseMcg = (lowerBound + upperBound) / 2; // Average for an estimate
resultDiv.innerHTML = "Estimated Starting Dose: " + estimatedDoseMcg.toFixed(2) + " mcg/day (based on weight)";
} else if (weightUnit === 'mcgPerM2') {
// Calculate BSA using Mosteller formula
var bsavalue = Math.sqrt((heightCm * weightKg) / 3600);
// Typical starting dose range: 100-150 mcg/m²/day
var lowerBoundBSA = bsavalue * 100;
var upperBoundBSA = bsavalue * 150;
estimatedDoseMcg = (lowerBoundBSA + upperBoundBSA) / 2; // Average for an estimate
resultDiv.innerHTML = "Estimated Starting Dose: " + estimatedDoseMcg.toFixed(2) + " mcg/day (based on BSA)";
}
resultDiv.innerHTML += "Note: This is an estimate. Always consult your healthcare provider.";
}